Methods and apparatus for adaptive compensation of signal bandwidth narrowing through finite impulse response filters

ABSTRACT

An apparatus includes a finite impulse response (FIR) filter to receive a digital signal and a transmitter, operatively coupled to the FIR filter, to transmit an analog signal, converted from the digital signal, to a communication channel. The FIR filer is configured to change at least one operating parameter based on a bandwidth of the analog signal after transmission in the communication channel. The bandwidth of the analog signal is estimated, using an estimator, based at least in part on raw sampling data generated by an analog-to-digital converter (ADC) operatively coupled to the transmitter.

CROSS REFERENCE TO RELATED APPLICATION

This patent application is a continuation of U.S. patent applicationSer. No. 15/395,753, (now U.S. Pat. No. 10,069,590), entitled “METHODSAND APPARATUS FOR ADAPTIVE COMPENSATION OF SIGNAL BANDWIDTH NARROWINGTHROUGH FINITE IMPULSE RESPONSE FILTERS,” filed Dec. 30, 2016, which isincorporated herein by reference.

FIELD

One or more embodiments relate to methods and apparatus of compensatingsignal bandwidth narrowing in optical communication systems.

BACKGROUND

Coherent line-cards using polarization-multiplexed quadraturephase-shift keying (PM-QPSK) or polarization-multiplexed quadratureamplitude modulation (PM-QAM) has become de-facto standards in quest forhigh spectral efficiency optical fiber communications systems. When amodulated signal passes through multiple reconfigurable optical add dropmultiplexers (ROADMs), the bandwidth of the propagating signal usuallynarrows down due to the spectral shape of the ROADM filter and due topossible misalignments of the central frequencies of the signal and theROADM pass-band. This bandwidth narrowing can in turn increase theobserved bit error ratio (BER) at the receiving end and cause thereceiver to lose track of the signal. Most dynamic gain equalizers(DGEs) can also introduce bandwidth narrowing by flattening the gainspectrum of cascaded Erbium-doped fiber amplifiers (EDFAs). Thebandwidth narrowing effect due to ROADMs and DGEs is relatively dynamic.

In addition, photodiode, trans-impedance amplifier, circuit traces,connector for pluggable optics, and analog digital converters (ADCs)form an analog interface between coherent receiver and digital signalprocessing (DSP) chip. There is also a similar analog interface betweenthe DSP chip and the coherent transmitter. Both analog interfaces caninfluence the signal bandwidth. Although this influence can berelatively static, it might degrade over the lifetime. Accordingly, aneed exists for methods and apparatus that compensate for the bandwidthnarrowing effect.

SUMMARY

Some embodiments described herein relate generally to compensation forbandwidth narrowing, and, in particular, to methods and apparatus foradaptive compensation of signal bandwidth narrowing in fibercommunication systems using finite impulse response (FIR) filters.

In some embodiments, an apparatus includes a finite impulse response(FIR) filter to receive a digital signal and a transmitter, operativelycoupled to the FIR filter, to transmit an analog signal, converted fromthe digital signal, to a communication channel. The FIR filer isconfigured to change at least one operating parameter based on abandwidth of the analog signal after transmission in the communicationchannel. The bandwidth of the analog signal is estimated, using anestimator, based at least in part on raw sampling data generated by ananalog-to-digital converter (ADC) operatively coupled to thetransmitter.

In some embodiments, a method includes transmitting a digital signalthrough a finite impulse response (FIR) filter converting the digitalsignal, after the FIR filter, into an analog signal. The method alsoincludes transmitting the analog signal to a communication channel usinga transmitter changing at least one operating parameter of the FIRfilter based on a bandwidth of the analog signal after transmission inthe communication channel. The bandwidth is estimated, using anestimator, based at least in part on raw sampling data generated by ananalog-to-digital converter (ADC) operatively coupled to thetransmitter.

In some embodiments, a bi-directional link includes a first FIR filterto receive a digital signal and a digital-to-analog converter (DAC),operatively coupled to the first FIR filter, to convert the digitalsignal to an analog signal. The bi-directional link also includes atransmitter, operatively coupled to the DAC, to transmit the analogsignal to a communication channel. A receiver is operatively coupled tothe transmitter, to receive the analog signal after the communicationchannel. An ADC is operatively coupled to the receiver to generate rawsampling data representative of the analog signal. The bi-directionallink also includes an estimator, operatively coupled to the ADC and thefirst FIR filter, to estimate a bandwidth of the analog signal based atleast in part on the raw sampling data and a second FIR filteroperatively coupled to the ADC and the estimator. The first FIR filterand the second FIR filter are configured to decrease a peaking frequencyand a peaking amplitude the first FIR filter and the second FIR filterin response to the bandwidth of the analog signal greater than apredetermined value.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic of an apparatus for adaptive compensation ofsignal bandwidth narrowing in a communication system, according toembodiments.

FIG. 2 shows a schematic of a coherent transceiver including finiteimpulse response (FIR) filters for adaptive compensate of signalbandwidth narrowing, according to embodiments.

FIG. 3 shows a schematic of a bi-directional coherent transceiver systemincluding FIR filters for adaptive compensation of signal bandwidthnarrowing, according to embodiments.

FIG. 4 shows a schematic of system using a network management device toprovide bandwidth information for FIR filters to adaptively compensatefor signal bandwidth narrowing, according to embodiments.

FIG. 5 shows a schematic of a system using in-band signals to providebandwidth information for FIR filters to adaptively compensate forsignal bandwidth narrowing, according to embodiments.

FIG. 6 illustrates a method of adaptive compensation for signalbandwidth narrowing using FIR filters in a communication system,according to embodiments.

FIG. 7 illustrates a method of adaptive compensation for signalbandwidth narrowing using FIR filters in a coherent optical transceiver,according to embodiments.

FIG. 8 shows a schematic of a system to characterize the techniques ofadaptive compensation for signal bandwidth narrowing in a communicationsystem, according to embodiments.

FIGS. 9A-9C show examples of experimental results of spectrum estimationusing digital filtering in a communication system.

FIG. 10 shows examples of measured attenuation at Nyquist frequency as afunction of receiver bandwidth in a coherent optical communicationsystem.

FIG. 11 shows examples of calculated magnitude response to different FIRfilters that can be used for adaptive compensation of bandwidthnarrowing.

FIG. 12 shows examples of measured bit error rate versus signal gain ofFIR filters that can be used for adaptive compensation of bandwidthnarrowing

FIG. 13 shows examples of experimental results of Q2 penalty as afunction of receiver bandwidth with different impairments loaded to thecommunication channel.

FIG. 14 shows examples of measured signal spectra under differentpeaking amplitudes of a transmitter FIR filter.

FIG. 15 shows examples of measured signal spectra under differentroll-off factor α of a transmitter FIR filter.

DETAILED DESCRIPTION

In some embodiments, an apparatus includes a finite impulse response(FIR) filter to receive a digital signal and a transmitter, operativelycoupled to the FIR filter, to transmit an analog signal, converted fromthe digital signal, to a communication channel. The FIR filter isconfigured to change at least one operating parameter based on abandwidth of the analog signal after transmission in the communicationchannel so as to proactively compensate for signal bandwidth narrowingin the communication channel. The bandwidth of the analog signal isestimated, using an estimator, based at least in part on raw samplingdata generated by an analog-to-digital converter (ADC) operativelycoupled to the transmitter.

In some embodiments, the ADC is disposed on the receiver end to generatethe raw sampling date representative of the bandwidth of the analogsignal. In some embodiments, the ADC can be the ADC already integratedin digital signal processing (DSP) chips. In this case, the apparatustakes advantage of existing hardware in current communication systems toadaptively control the bandwidth of signals.

In some embodiments, the FIR filter compensates for the signal bandwidthnarrowing by changing its peak frequency and/or peak amplitude. In someembodiments, the FIR filter decreases at least one of the peakingfrequency or the peaking amplitude in response to the bandwidth of theanalog signal greater than a predetermined value. In some embodiments,the FIR filter increases at least one of the peaking frequency or thepeaking amplitude in response to the bandwidth of the analog signalsmaller than a predetermined value.

The apparatus can be implemented in various communication systems. Insome embodiments, the apparatus can be employed in a bi-directionalcommunication system, in which the bandwidth of the analog signal can bedirectly fed to the FIR included in the same bi-directional transceiverthat includes the ADC. In some embodiments, the FIR is configured toreceive, via an in-band signal, a control signal based on the bandwidthof the analog signal to change the at least one operating parameter. Insome embodiments, the FIR filter is configured to receive, via a networkmanagement device, a control signal based on the bandwidth of the analogsignal to change the at least one operating parameter.

In some embodiments, a method includes transmitting a digital signalthrough a finite impulse response (FIR) filter converting the digitalsignal, after the FIR filter, into an analog signal. The method alsoincludes transmitting the analog signal to a communication channel usinga transmitter changing at least one operating parameter of the FIRfilter based on a bandwidth of the analog signal after transmission inthe communication channel. The bandwidth is estimated, using anestimator, based at least in part on raw sampling data generated by ananalog-to-digital converter (ADC) operatively coupled to thetransmitter.

FIG. 1 shows a schematic of an apparatus 100 for adaptive compensationof signal bandwidth narrowing in a communication system, according toembodiments. The apparatus 100 includes a FIR filter 110 to receive adigital signal 101 and a transmitter 120, operatively coupled to the FIRfilter 110, to transmit an analog signal 102 (e.g., an optical signal),converted from the digital signal 101, to a communication channel 130.The communication channel 130 can include ROADMs or other componentsthat can cause bandwidth narrowing of the analog signal 102. After thecommunication channel 130, the analog channel 102 is received by an ADC140, which converts the analog signal 102 back into a digital signal.During the conversion, the ADC 140 generates raw sampling data that canbe representative of the spectrum of the analog signal 102. For example,an 8-bit ADC can generate raw sampling data that represents valuesbetween −128 to 127 and is proportional to the analog signal 102.

An estimator 150 is operatively coupled to the ADC and the FIR filter110 (thereby operatively coupled to the transmitter 120 as well) toreceive the raw sampling data from the ADC 140 and estimate thebandwidth of the analog signal 102 based on the raw sampling data. TheFIR filter 110 is configured to change at least one operating parameterbased on the bandwidth of the analog signal 102 after transmission inthe communication channel 130. This change of operating parameter of theFIR filter 110 can compensate for bandwidth narrowing that occurs in thecommunication channel 130. Because the digital signal 101 is transmittedthrough the FIR filter 110 before being converted into the analog signal102 and transmitted through the communication channel, this compensationis also referred to as pre-compensation, or transmitter-endcompensation. Since the compensation is based on only raw sampling datafrom the ADC 140, this technique does not require DSP lock or recoveryof the digital signal. This can be especially advantageous when thecommunication channel 130 introduces significant amount of narrowing tothe analog signal 102 such that recovering the digital signal from theanalog signal 102 can be challenging.

In some embodiments, the FIR filter 110 and the ADC 140 are built in aDSP chip and the parameters of the FIR filter 110 can be controlled byadjusting registers within the DSP chip. At the same time, a short sizeof the raw sampling data of the ADC 140 can be stored in the on-chipmemory of the DSP chip. The estimator 150 can include a controller (notshown in FIG. 1) to extract the raw sampling data from the DSP chip,estimate the bandwidth, and then configure FIR filter 110 accordingly.

The FIR filter 110 can change at least one of the peaking frequency, thepeaking amplitude, and/or the roll-off factor to compensate for thebandwidth narrowing of the analog signal 102. In general, the shape ofthe FIR filter 110 can be inversely proportional to the analog signal102 for compensation purposes.

In some embodiments, the FIR filter 110 can change its peaking frequencybased on the bandwidth information received from the estimator 150. Forexample, the FIR filter 110 can decrease the peaking frequency inresponse to the bandwidth of the analog signal 102 greater than apredetermined value. Alternatively, the FIR filter 110 can increase thepeaking frequency in response to the bandwidth of the analog signal 102less than a predetermined value. In other words, the FIR filter 110adjusts its peaking frequency based on absolute values of the bandwidthof the analog signal 102. In some embodiments, the predetermined valuecan be about 10 GHz to about 30 GHz (e.g., about 10 GHz, about 15 GHz,about 20 GHz, about 25 GHz, or about 30 GHz, including any values andsub ranges in between).

In some embodiments, the FIR filter 110 can change its peaking frequencybased on the change of the bandwidth of the analog signal 102. Forexample, the estimator 150 can monitor the bandwidth of the analogsignal 102 in a substantially real-time manner. The measurement of thebandwidth can be performed at an acquisition frequency substantiallyequal to or greater than 10 Hz (e.g., about 10 Hz, about 15 Hz, about 20Hz, about 30 Hz, about 50 Hz, or greater, including any values and subranges in between). In response to a decrease of the detected bandwidth,the FIR filter 110 can increase its peaking frequency. In response to anincrease of the detected bandwidth, the FIR filter 110 can decrease itspeaking frequency. Stated differently, the FIR filter 110 adjusts thepeaking frequency based on a relative change of the bandwidth of theanalog signal 102.

The peaking frequency of the FIR filter 110 can be changed within arange up to the Nyquist frequency of the analog signal 102. In someembodiments, the peaking frequency of the FIR filter 110 can be about10% to about 100% of the Nyquist frequency of the analog signal (e.g.,about 10%, about 20%, about 30%, about 40%, about 50%, about 60%, about70%, about 80%, about 90%, about 95%, about 99%, or about 100%,including any values and sub ranges in between).

In some embodiments, the FIR filter 110 can change its peaking amplitudebased on the bandwidth information received from the estimator 150. Forexample, the FIR filter 110 can decrease the peaking amplitude inresponse to the bandwidth of the analog signal 102 greater than apredetermined value. Alternatively, the FIR filter 110 can increase thepeaking amplitude in response to the bandwidth of the analog signal 102less than a predetermined value. In other words, the FIR filter 110adjusts its peaking amplitude based on absolute values of the bandwidthof the analog signal 102. In some embodiments, the predetermined valueof the bandwidth can be about 10 GHz to about 30 GHz (e.g., about 10GHz, about 15 GHz, about 20 GHz, about 25 GHz, or about 30 GHz,including any values and sub ranges in between).

In some embodiments, the FIR filter 110 can change its peaking amplitudebased on the change of the bandwidth of the analog signal 102. Forexample, the estimator 150 can monitor the bandwidth of the analogsignal 102 in a substantially real-time manner. In response to adecrease of the detected bandwidth, the FIR filter 110 can increase itspeaking amplitude. In response to an increase of the detected bandwidth,the FIR filter 110 can decrease its peaking amplitude. Stateddifferently, the FIR filter 110 adjusts the peaking amplitude based on arelative change of the bandwidth of the analog signal 102.

The peaking amplitude of the FIR filter 110 can be changed within arange of about 1 dB to about 9 dB. For example, the peaking amplitude ofthe FIR filter 110 can be about 1 dB, about 2 dB, about 3 dB, about 4dB, about 5 dB, about 6 dB, about 7 dB, about 8 dB, or about 9 dB,including any values and sub ranges in between.

In some embodiments, the FIR filter 110 can change its roll-off factorbased on the bandwidth information received from the estimator 150. Forexample, the FIR filter 110 can decrease the roll-off factor in responseto the bandwidth of the analog signal 102 less than a predeterminedvalue. Alternatively, the FIR filter 110 can increase the roll-offfactor in response to the bandwidth of the analog signal 102 greaterthan a predetermined value. In other words, the FIR filter 110 adjustsits roll-off factor based on absolute values of the bandwidth of theanalog signal 102. In some embodiments, the predetermined value of thebandwidth can be about 10 GHz to about 30 GHz (e.g., about 10 GHz, about15 GHz, about 20 GHz, about 25 GHz, or about 30 GHz, including anyvalues and sub ranges in between).

In some embodiments, the FIR filter 110 can change its roll-off factorbased on the change of the bandwidth of the analog signal 102. Forexample, the estimator 150 can monitor the bandwidth of the analogsignal 102 in a substantially real-time manner. In response to adecrease of the detected bandwidth, the FIR filter 110 can decrease itsroll-off factor. In response to an increase of the detected bandwidth,the FIR filter 110 can decrease its roll-off factor. Stated differently,the FIR filter 110 adjusts the roll-off factor based on a relativechange of the bandwidth of the analog signal 102.

The roll-off factor of the FIR filter 110 (such as a raise-cosinefilter) can be changed within a range of about 0.1 to about 1. Forexample, the roll-off factor of the FIR filter 110 can be about 0.1,about 0.2, about 0.3, about 0.4, about 0.5, about 0.6, about 0.7, about0.8, about 0.9, or about 1, including any values and sub ranges inbetween.

In some embodiments, the FIR filter 110 can change more than oneparameter for adaptive compensation. For example, the FIR filter 110 canchange both the peaking frequency and the peaking amplitude based on thebandwidth of the analog signal 102. In another example, the FIR filter110 can change the peaking frequency, the peaking amplitude, and theroll-off factor based on the bandwidth of the analog signal 102.

The transmitter 120 can include one or more types of transmitters. Insome embodiments, the transmitter 120 can include a coherenttransmitter. In some embodiments, the transmitter 120 can include aC-form pluggable generation (CFP) transmitter. In some embodiments, thetransmitter 120 can include a CFP4 transmitter, which can be coherent ornon-coherent. In some embodiments, the transmitter 120 can include aC-form pluggable generation 2-analog coherent optics (CFP2-ACO)transmitter that is coupled with a digital signal processor (DSP) chipthrough a pluggable interface.

In some embodiments, the transmitter 120 can include a coherentin-phase/quadrature transmitter integrated together with a DSP within aphysical module. In some embodiments, the transmitter 120 can include aC-form pluggable generation—digital coherent optics (CFP-DCO)transmitter integrated with a DSP and an optical front end. In someembodiments, the transmitter 120 can include a Quad Small Form-factorPluggable (QSFP) transmitter. In some embodiments, the transmitter 120can include a QSFP28 transmitter. These transmitters can be coherent orincoherent.

The estimator 150 can estimate the bandwidth of the analog signal 102based on the raw sampling data of the ADC 140 via various methods. Insome embodiments, the estimator 150 is configured to perform a Fouriertransform on the raw sampling data to compute the spectrum of the analogsignal 102 and then estimate the bandwidth of the analog signal 102. Insome embodiments, the Fourier transform can be carried out via fastFourier transform (FFT) methods or any other digital methods known inthe art.

In some embodiments, the estimator 150 is configured to perform digitalfiltering before estimating the bandwidth of the analog signal. Forexample, the estimator 150 can be configured to perform Fouriertransform of the raw ADC data to generate unfiltered spectrum. Then theestimator 150 performs a digital filtering to the unfiltered spectrum togenerate filtered data and estimates the bandwidth of the analog signalbased on the filtered data. In some embodiments, the digital filteringcan be Savitzky-Golay filtering. In some embodiments, the digitalfiltering can be moving average filtering, or any other appropriatedigital filtering techniques.

The bandwidth information estimated by the estimator 150 can be providedto the FIR filter 110 via various channels. In some embodiments, the FIRfilter 110 can be part of a bi-directional link, which also includes theADC 140. In this case, the bandwidth information estimated from the ADC140 can be transmitted to the FIR filter 110 (see, e.g. FIG. 3) via, forexample, on-chip communication. In some embodiments, the bandwidthinformation can be transmitted to the FIR filter 110 via a networkmanagement device (see, e.g., FIG. 4) via transmission channels outsidethe module containing the ADC 140 and/or the FIR filter 110. In someembodiments, the bandwidth information can be transmitted to the FIRfilter 110 via in-band signals (see, e.g., FIG. 5) within, for example,the DSP chip containing the ADC 140.

In some embodiments, a control signal is first generated based on thebandwidth of the analog signal 102 and then transmitted to the FIRfilter 110 to adjust its operating parameters. In some embodiments, thecontrol signal can be generated by the estimator 150 and transmitted tothe FIR filter 110 via any of the methods described above (e.g., vianetwork management device, via in-band signal, or via directtransmission). In some embodiments, the control signal can be generatedby the DSP chip, into which the FIR filter 110 is integrated.

In some embodiments, the apparatus 100 can include a second FIR filter(not shown in FIG. 1) disposed after the ADC 140 (e.g., on the receiverend). The bandwidth of the analog signal 102 can also be employed tocontrol operating parameters of the second FIR filter, in a mannersimilar to the control of the FIR filter 110. Using two FIR filters (oneon the transmitter end and the other on the receiver end) can furtherimprove the tolerance to signal bandwidth narrowing in the communicationchannel 130.

In some embodiments, the communication channel 130 includes a fibercommunication channel, which in turn includes multiple ROADMs that canintroduce bandwidth narrowing to the analog signal 102. In someembodiments, the communication channel 130 can include semiconductorwaveguides with directional couplers as add/drop couplers. In someembodiments, the communication channel 130 can include any other type ofchannels known in the art.

FIG. 2 shows a schematic of a coherent transceiver 200 including FIRfilters for adaptive compensate of signal bandwidth narrowing, accordingto embodiments. The transceiver 200 includes a transmitter module 210 totransmit an analog signal into a communication channel 220, and areceiver module 230 to receive the analog signal after transmission inthe communication channel 220. The transmitter module 210 furtherincludes a transmitter end forward error correction (FEC) device 212, atransmitter FIR filter 214, a digital-to-analog converter (DAC) 216, anda transmitter 218 (e.g., a CFP2-ACO transmitter). The communicationchannel 220 includes two groups of ROADMs 220 a and 220 b, which canintroduce bandwidth narrowing to the analog signal propagating in thecommunication channel 220. The communication channel 220 can alsointroduce other impairments, such as chromatic dispersion (CD),polarization mode dispersion (PMD), and polarization dependent loss(PDL), among others. The receiver module 230 includes a receiver 231(e.g., a CFP2-ACO receiver), an ADC 232, a receiver FIR filter 233, a CDcompensator 234, a PMD compensator 235, a carrier phase estimation (CPE)compensator 236, and a receiver end FEC device 237.

In operation, the ADC 232 in the receiver module 230 converts the analogsignal transmitted through the communication channel 220 into a digitalsignal. The raw sampling data of the ADC 232 can be used to estimate thespectrum and therefore bandwidth of the analog signal. This bandwidthinformation can then be employed as the basis to control the transmitterend FIR filter 214 and/or the receiver end FIR filter 233 to adjusttheir operating parameters, such as peaking frequency, peakingamplitude, and roll-off factor. This change of operating parameter(s)can in turn compensate for the bandwidth narrowing introduced by thecommunication channel 220, in particular the ROADMs.

Because the receiver end FIR filter 233 can be generally coupled to theADC 232 within the same physical module, bandwidth information of theanalog signal can be directly transmitted from the ADC 232 (or anyestimator coupled to the ADC) to the receiver end FIR filter 233. Incontrast, the transmitter end FIR filter 214 may be physically separateor even remote from the ADC 232. Accordingly, the communication betweenthe ADC 232 and the transmitter end FIR filter 214 can be based on thetype of transceiver 200. Some illustrative examples of suchcommunication are shown in FIGS. 3-5 and described below.

FIG. 3 shows a schematic of a bi-directional coherent transceiver system300 including FIR filters for adaptive compensation of signal bandwidthnarrowing, according to embodiments. The transceiver system 300 includesa first node 301 on the west end (left end) and a second node 302 on theeast end (right end) separated by a communication channel 320.

The first node 301 includes a transmitter module 310, a receiver module360, and a bandwidth estimator 370. The transmitter module 310 includesa transmitter end FEC device 312, a transmitter FIR filter 314, a DAC316, and a transmitter 318. The receiver module 360 of the first node301 includes a receiver 361 (e.g., a CFP2-ACO receiver), an ADC 362, areceiver FIR filter 363, a CD compensator 364, a PMD compensator 365, aCPE compensator 366, and a receiver end FEC device 367. The bandwidthestimator 370 is operatively coupled to the ADC 362 to estimatebandwidth of the received analog signal based on raw sampling datagenerated by the ADC 362. The bandwidth estimator 370 is furtheroperatively coupled to the transmitter FIR filter 314 and the receiverFIR filter 363 so as to control the two FIR filters 314 and 363 forbandwidth narrowing compensation.

The second node 302 can have a substantially similar diagram to thefirst node 301. The second node 302 also includes a transmitter module350, a receiver module 330, and a bandwidth estimator 340. Thetransmitter module 350 further includes a transmitter end FEC device352, a transmitter FIR filter 354, a DAC 356, and a transmitter 358. Thereceiver module 330 of the second node 302 includes a receiver 331(e.g., a CFP2-ACO receiver), an ADC 332, a receiver FIR filter 333, a CDcompensator 334, a PMD compensator 335, a CPE compensator 336, and areceiver end FEC device 337. The bandwidth estimator 340 is operativelycoupled to the ADC 332 to estimate bandwidth of the received analogsignal based on raw sampling data generated by the ADC 332. Thebandwidth estimator 340 is further operatively coupled to thetransmitter FIR filter 354 and the receiver FIR filter 333 so as tocontrol the two FIR filters 354 and 333 for bandwidth narrowingcompensation.

In some embodiments, the communication between the first node 301 andthe second node 302 can be bi-directional. In this case, the transmittermodule 310 of the first node 301 is in communication with the receivermodule 330 of the second node 302, while the transmitter module 350 ofthe second node 302 is in communication with the receiver module 360 ofthe first node 301. The links in both directions include substantiallythe same number of ROADMs. In addition, the ROADMs in both directionsare usually of the same type, from the same vendor, and in the sameambient environment. Therefore, it can be assumed that the effect ofbandwidth narrowing in the two directions (i.e., from the first node 301to the second node 302 and from the second node 302 to the first node301) is also substantially the same.

With the above assumption, in some embodiments, one can estimatebandwidth from the receiver module 330 in the second node 302, and thenuse the estimated bandwidth to adjust the transmitter FIR 354 in thesame node (i.e., second node 302). This adjustment can improvecharacteristics of the east-west link (i.e. data transmission from thesecond node 302 to the first node 301).

In some embodiments, one can estimate bandwidth from the receiver module360 in the first node 301 and then use the estimated bandwidth to adjustthe transmitter FIR 314 in the first node 301. This adjustment canimprove characteristics of the west-east link (i.e., data transmissionfrom the first node 301 to the second node 302).

FIG. 4 shows a schematic of system 400 using a network management deviceto provide bandwidth information for FIR filters to adaptivelycompensate for signal bandwidth narrowing, according to embodiments. Thesystem 400 includes a first transmitter module 410 and a first receivermodule 430 separated by a communication channel 420. The system 400 alsoincludes a second transmitter module 450 and a second receiver module460 also separated by the communication channel 420. Unlike thebi-directional communication shown in FIG. 3, however, the communicationfrom the first transmitter module 410 to the first receiver module 430can be different from the communication from the second transmittermodule 450 to the second receiver module 460. For example, thecommunications on these two directions can include different numbers ofROADMs and/or different types of ROADMs. As a result, the bandwidthnarrowing effects on these two directions can be different.

The first transmitter module 410 includes a FEC device 412, atransmitter FIR filter 414, a DAC 416, and a transmitter 418. The firstreceiver module 430 includes a receiver 431 (e.g., a CFP2-ACO receiver),an ADC 432, a receiver FIR filter 433, a CD compensator 434, a PMDcompensator 435, a CPE compensator 436, and a receiver-end FEC device437.

Similarly, the second transmitter module 450 also includes a FEC device452, a transmitter FIR filter 454, a DAC 456, and a transmitter 458. Thesecond receiver module 460 includes a receiver 461 (e.g., a CFP2-ACOreceiver), an ADC 462, a receiver FIR filter 463, a CD compensator 464,a PMD compensator 465, a CPE compensator 466, and a receiver-end FECdevice 467.

A bandwidth estimator 470 is operatively coupled to the ADC 462 in thesecond receiver module 460 to estimate bandwidth of the analog signaltransmitted by the second transmitter module 450 and received by thesecond receiver module 460. The estimated bandwidth is then transmittedto a network management device 480, which in turn transmits thebandwidth information to the transmitter FIR filter 454 to compensatefor bandwidth narrowing that occurs in the communication channel fromthe second transmitter module 450 to the second receiver module 460.

Similarly, another bandwidth estimator (not shown in FIG. 4) can beoperatively coupled to the ADC 432 in the first receiver module 430 andestimate the bandwidth of received analog signals. The estimatedbandwidth can be then transmitted to the transmitter FIR filter 414 inthe first receiver module 410 for bandwidth narrowing compensation viathe network management device 480. In this manner, the characteristicsof the communication between the first transmitter module 410 and thefirst receiver module 430 can be improved.

In some embodiments, the estimated bandwidth from the bandwidthestimator 470 can also be provided to the receiver FIR filter 463 in thesecond receiver module 460. As discussed above, using two FIR filters(one on the transmitter end and the other on the receiver end) canfurther improve the tolerance to signal bandwidth narrowing in thecommunication channel 420.

FIG. 5 shows a schematic of a system 500 using in-band signals toprovide bandwidth information for FIR filters. The system 500 includes afirst node 501 on the west end (left end) and a second node 502 on theeast end (right end) separated by a communication channel 520.

The first node 501 includes a transmitter module 510, a receiver module560, and a bandwidth estimator 570. The transmitter module 510 includesa transmitter end FEC device 512, a transmitter FIR filter 514, a DAC516, and a transmitter 518. The receiver module 560 of the first node501 includes a receiver 561 (e.g., a CFP2-ACO receiver), an ADC 562, areceiver FIR filter 563, a CD compensator 564, a PMD compensator 565, aCPE compensator 566, and a receiver end FEC device 567. The bandwidthestimator 570 is operatively coupled to the ADC 562 to estimatebandwidth of the received analog signal based on raw sampling datagenerated by the ADC 562. The bandwidth estimator 570 is furtheroperatively coupled to the transmitter module 510 via in-band signals.

The second node 502 can have a substantially similar diagram to thefirst node 501. The second node 502 also includes a transmitter module550, a receiver module 530, and a bandwidth estimator 540. Thetransmitter module 550 further includes a transmitter end FEC device552, a transmitter FIR filter 554, a DAC 556, and a transmitter 558. Thereceiver module 530 of the second node 502 includes a receiver 531(e.g., a CFP2-ACO receiver), an ADC 532, a receiver FIR filter 533, a CDcompensator 534, a PMD compensator 535, a CPE compensator 536, and areceiver end FEC device 537. The bandwidth estimator 540 is operativelycoupled to the ADC 332 to estimate bandwidth of the received analogsignal based on raw sampling data generated by the ADC 532. Thebandwidth estimator 540 is further operatively coupled to thetransmitter module 550 via in-band signals.

In some embodiments, the in-band signals can be embedded in an IPpacket(s) in the reverse direction. In some embodiments, the in-bandsignals can be embedded in optical transport network (OTN) frame in thereverse direction. For example, the receiver module 530 in the secondnode 502 can estimate the signal bandwidth in west-east direction (i.e.,from the first node 501 to the second node 502). This estimatedbandwidth can be then sent through the transmitter module 550 in thesecond node 502, which transmits signals in east-west direction (i.e.,from second node 502 to first node 501). Once the first node 501receives the bandwidth information, it can adaptively adjust thetransmitter FIR filter 514 for pre-compensation, which improves theperformance of west-east link.

In another example, the receiver module 560 in the first node 501 canestimate the signal bandwidth in east-west direction (i.e., from thesecond node 502 to the first node 501). This estimated bandwidth can bethen sent through the transmitter module 510 in the first node 501,which transmits signals in west-east direction (i.e., from first node501 to second node 502). Once the second node 502 receives the bandwidthinformation, it can adaptively adjust the transmitter FIR filter 554 forpre-compensation, which improves the performance of east-west link.

In some embodiments, the estimated bandwidth from the bandwidthestimator 540 can also be provided to the receiver FIR filter 533 in thefirst receiver module 530. As discussed above, using two FIR filters(one on the transmitter end and the other on the receiver end) canfurther improve the tolerance to signal bandwidth narrowing in thecommunication channel 520.

Similarly, in some embodiments, the estimated bandwidth from thebandwidth estimator 570 can also be provided to the receiver FIR filter563 in the second receiver module 560. As discussed above, using two FIRfilters (one on the transmitter end and the other on the receiver end)can further improve the tolerance to signal bandwidth narrowing in thecommunication channel 520.

FIG. 6 illustrates a method 600 of adaptive compensation for signalbandwidth narrowing using FIR filters in a communication system,according to embodiments. The method 600 includes transmitting a digitalsignal through a finite impulse response (FIR) filter at step 610. Thedigital signal is then converted into an analog signal after the FIRfilter, at step 620. At step 630, the analog signal is transmitted to acommunication channel using a transmitter. The method 600 also includeschanging at least one operating parameter of the FIR filter based on thebandwidth of the analog signal after transmission in the communicationchannel at step 640. The bandwidth of the analog signal is estimated,using an estimator, based at least in part on raw sampling datagenerated by an analog-to-digital converter (ADC) operatively coupled tothe transmitter.

In some embodiments, the FIR filter changes at least one of the peakingfrequency, the peaking amplitude, or the roll-off factor so as tocompensate for bandwidth narrowing in the communication channel. In someembodiments, the FIR filter decreases the peaking frequency in responseto the bandwidth of the analog signal greater than a predeterminedvalue. In some embodiments, the FIR filter increases the peakingfrequency in response to the bandwidth of the analog signal smaller thana predetermined value.

In some embodiments, the FIR filter decreases the peaking amplitude inresponse to the bandwidth of the analog signal greater than apredetermined value. In some embodiments, the FIR filter increases thepeaking amplitude in response to the bandwidth of the analog signalsmaller than a predetermined value.

In some embodiments, the FIR filter decreases the roll-off factor inresponse to the bandwidth of the analog signal less than a predeterminedvalue. In some embodiments, the FIR filter increases the roll-off factorin response to the bandwidth of the analog signal greater than apredetermined value.

In some embodiments, the FIR filter decreases the peaking frequency inresponse to an increase of the bandwidth of the analog signal. In someembodiments, the FIR filter increases the peaking frequency in responseto a decrease of the bandwidth of the analog signal.

In some embodiments, the FIR filter decreases the peaking amplitude inresponse to an increase of the bandwidth of the analog signal. In someembodiments, the FIR filter increases the peaking amplitude in responseto a decrease of the bandwidth of the analog signal.

In some embodiments, the FIR filter decreases the roll-off factor inresponse to a decrease of the bandwidth of the analog signal. In someembodiments, the FIR filter increases the roll-off factor in response toan increase of the bandwidth of the analog signal.

In some embodiments, the estimator estimates the bandwidth of the analogsignal by performing Fourier transform of the raw ADC data to generateunfiltered spectrum. The estimator then performs digital filtering tothe unfiltered spectrum to generate filtered data and estimate thebandwidth of the analog signal based on the filtered data.

In some embodiments, the digital filtering includes Savitzky-Golayfiltering. In some embodiments, the digital filtering includes movingaverage filtering to the unfiltered spectrum to generate filtered data.

In some embodiments, the FIR filter receives the bandwidth informationvia a network management device. In some embodiments, the FIR filterreceives the bandwidth information via an in-band signal. In someembodiments, the FIR filter and the ADC are included in the same module(e.g., a DSP chip) and the FIR filter receives the bandwidth informationfrom on-chip communication.

FIG. 7 illustrates a method 700 of adaptive compensation for signalbandwidth narrowing using FIR filters in a coherent optical transceiver,according to embodiments. The method includes loading a transmitter FIRfilter and a receiver FIR filter with default tap coefficients tocompensate for the channel loss of RF traces, at 710. RF traces usuallyrefer to print circuit board (PCB) traces connecting between DSP andcoherent optics. Signals on RF traces are typically radio frequencysignals. These signals can experience certain loss when propagatingalong the trace. Generally, a higher frequency of the propagating signalcan lead to more losses.

At 720, the bandwidth of an analog signal after transmission in thechannel is estimated. The FIR filter on the receiver-end can also beadaptively adjusted based on the estimated bandwidth. This initialcompensation can be mainly for the channel loss of RF trace and otherstatic effects that cause bandwidth narrowing. Without thispre-compensation, it can be challenging for the signal to reach theother end of the communication channel. At this step, somepre-compensation is carried out and the signal bandwidth is estimated.Then the estimated bandwidth information is used in subsequent steps toadaptively adjust the FIR filter to compensate for bandwidth narrowingdue to the dynamic effect, such as bandwidth narrowing introduced bymultiple ROADMs.

At 730, the optical spectrum of the transmitter signal is estimatedbased on calibration, and the electrical spectrum of the receiver signalis estimated based on FFT of ADC raw data. Based on the optical spectrumof the transmitter signal and the electrical spectrum of the receiversignal, the bandwidth of the link (e.g., the bandwidth of thedegradation due to multiple ROADMs in the channel) is calculated.

At an indicator of 735, the type of the channel is detected. If thechannel is bidirectional symmetric, the bandwidth of the link is fedback to the transmitter directly and the FIR filter on the transmittercan be adjusted based on the bandwidth of the link accordingly to apre-calibrated table, at 740. If the channel is not bi-directional, thebandwidth information is sent to the transmitter FIR filter throughin-band signal or a network management device, at 750, after which thetransmitter FIR filter is adjusted accordingly, at 740.

At 760, the signal bandwidth is estimated at the receiver end. Ifbandwidth degradation is detected at 765, the method proceeds to 770,where the receiver FIR filter is adjusted based on the estimated signalbandwidth. The adjustment can also compensate any long-term drift anddegradation. If no bandwidth degradation is detected at 765, the methodproceeds to 775 to determine whether network rerouting happens. In anevent of network rerouting determined at 775, this procedure can berepeated (i.e., the method 700 proceeds back to 710). On the other hand,in the absence of network rerouting determined at 775, the methodsproceeds back to 760.

FIG. 8 shows a schematic of an example setup 800 for experimentalcharacterization of techniques described herein for adaptivecompensation of bandwidth narrowing. In particular, the setup 800 can beused to optimize tap coefficient for FIR filters under differentimpairments. The setup 800 includes a transmitter 810 (e.g., a class 1CFP2-ACO transceiver running PM-QPSK modulation format) to provide anincident signal. The setup 800 also includes emulators 830 to simulateseveral types of impairments in the transmission system. These emulators830 includes a CD emulator 830 a, a PMD emulator 830 b, and a PDLemulator 830 c. A 4×4 non-blocking optical switches 820 can load eitheran individual simulated impairment or any combination of simulatedimpairments in the optical path from the emulators 830. After loadingvarious impairments, the signal is combined with amplified spontaneousemission (ASE) noise, provided by an ASE source 842 noise andtransmitted through a tunable filter 844, so as to change opticalsignal-to-noise ratio (OSNR) of the signal.

The signal loaded with various impairments and ASE is received by acoupler 850, which couples part of the received signal to a tunableoptical filter 860 (also referred to as an optical filter here). Anoptical spectrum analyzer (OSA) 855 is operably coupled to the coupler850 to measure the optical signal noise ratio. The optical filter 860has tunable center frequency and pass-band and is placed in front ofcoherent receiver. By adjusting the pass-band of the optical filter 860,the impairment of receiver bandwidth narrowing can be emulated.

FIGS. 9A-9C show experimental results of spectrum and bandwidthestimation using digital filtering in a communication system. Theestimated bandwidth can be then used for optimizing the tap coefficientof the optical filter 860. FIG. 9A shows spectra from ADC raw outputs.FIG. 9B shows spectra after Savitzky-Golay filtering. FIG. 9C showsspectra of XI tributary under different bandwidth of tunable filter withOSNR=14 dB. Modern optical communication system usually uses phasemodulation. Signal can be coded in two orthogonal domains: in-phase (I)domain and quadrature (Q) domain. The communication system can also usetwo polarizations of light (i.e., X polarization and Y polarization).Thus there are 4 tributaries to carry data, XI, XQ, YI, and YQ. FIGS.9A-9C show only XI tributary for clarification, but the data from othertributaries can be substantially similar.

In some embodiments, coherent DSP ASIC can provide a snapshot of ADC rawdata. The spectrum of modulated signal can be obtained by performing afast Fourier transform (FFT), as shown in FIG. 9A. The modulation indata, however, can make it challenging to estimate bandwidth. To addressthis issue, a Savitzky-Golay filter can be applied to the spectragenerated by FFT. A Savitzky-Golay filter can improve signal-to-noiseratio with small distortion of signal. FIG. 9B shows the spectra (alsoreferred to as the filtered spectra) after Savitzky-Golay filter.

The bandwidth of the optical tunable filter 860 in the setup 800 can bethen adjusted to emulate the narrowing of signal bandwidth. The spectraof XI tributary under different bandwidth of tunable filter are shown inFIG. 9C. Three values of receiver bandwidth (i.e. 43.9 GHz, 34.1 GHz,24.3 GHz) are presented in FIG. 9C.

To quantify the signal bandwidth, a parameter AttnNyquist can be definedas the difference between one spectrum's edge (at Nyquist frequency) andthe spectrum's center:Attn_(Nyquist) =P _(center) −P _(Nyquist)

The AttnNyquist can be further averaged over 4 tributaries to improvethe accuracy. Physically, the attenuation at Nyquist frequency can beproportional to signal bandwidth and therefore can be used to quantifythe signal bandwidth.

FIG. 10 shows examples of attenuation at Nyquist frequency versusreceiver bandwidth. For each bandwidth shown in FIG. 10, ADC raw data isrecorded and attenuation at Nyquist frequency is calculated 16 times.FIG. 10 shows a roughly linear relationship between AttnNyquist and thesignal bandwidth at the receiver. Small variation exists in AttnNyquist,so multiple measurements can be used to improve accuracy. AlsoAttnNyquist is dependent on OSNR level. In general, a lower OSNR canlead to a higher attenuation at Nyquist frequency.

It can be seen from FIG. 10 that the center of noise spectrumexperiences much less loss than the edge of noise spectrum, which leadsto increases of AttnNyquist. To distinguish different cases, one can usethe pre-FEC BER for estimation of OSNR.

FIG. 11 shows examples of magnitude response to different FIR filtersthat can be used in apparatus and methods described herein for adaptivecompensation of bandwidth narrowing.

FIG. 12 shows examples of bit error rate versus signal gain of FIRfilters that can be used in apparatus and methods described herein foradaptive compensation of bandwidth narrowing.

Four sets of FIR filters sharing the same tap coefficients areimplemented in the coherent DSP chip. They are located after ADC andbefore frequency-domain CD compensation. The tap coefficients of thedigital filter are real numbers only. Two parameters determine the shapeof channel filters: peaking frequency and peaking amount, as shown inFIG. 11.

In addition, multiple sets of tap coefficients can satisfy therequirement for each filter shape. Then the gain of FIR filter,determined by summation of the absolute value of tap coefficients, canalso become relevant. As shown in FIG. 12, a high gain can lead tosignal being clipped, while a low gain can lead to signal beingsubmerged from noise. As a result, it can be beneficial in general tofind the optimal gain value for each filter. It is possible tocompensate skew and imbalance among tributaries with different tapcoefficients for each tributary channel.

FIG. 13 shows examples of Q2 penalty as a function of receiver bandwidthwith different impairments, OSNR=16 dB, and receiver optical power=−18dBm. The back-to-back curve with 4 dB peaking at 19 GHz is used as thebaseline in calculating Q2 penalty.

In general, the shape of the filter 860 can be inversely proportional tothe signal for equalization (i.e. compensation) purpose. When the signalbandwidth is high, the optimal filter can have small peaking amplitudeand small peaking frequency. Otherwise, more noise than signal can beamplified, thereby generating the net effect of amplifying noises. Onthe other hand, when the signal bandwidth is low, the optimal filter canhave large peaking amplitude and large peaking frequency. In this way,the reduction of signal bandwidth can be compensated by adaptive FIRfilter.

As seen in FIG. 13, the experimental results agree with theoreticalanalysis above. For example, the FIR filter with 2 dB peaking at 13 GHzperforms better when the receiver bandwidth (RBW) is greater than 40GHz. In another example, the FIR filter with 4 dB peaking at 19 GHzperforms better when the RBW is less than 30 GHz. In the scenarios with3 dB PDL, combined impairment (e.g., CD+PMD+PDL), and back-to-back, thelockable range (i.e., the range of frequencies at which the coherentreceiver can recover digital data from the channel) of receiver with 4dB peaking at 19 GHz (as shown in diamond symbol) is about 5 GHz largerthan receiver with 2 dB peaking at 15 GHz. This can allow signal to passmore than 10 additional ROADMs.

FIG. 14 shows examples of measured signal spectra under differentpeaking amplitudes of an FIR filter. FIG. 15 shows measured signalspectra under different roll-off factor α of an FIR filter.

As discussed above, operating parameters for FIR filters (including bothtransmitter FIR filters and receiver FIR filters) include the peakingamplitude, the peaking frequency, and the roll-off factor. FIG. 14 showsthe optical spectra with different peaking amplitudes and FIG. 15 showsoptical spectra with different roll-off factors which are thecoefficients for raised-cosine function. As seen, transmitter FIRfilters can allow great capability and flexibility for pre-compensationof bandwidth narrowing effect. In general, with bandwidth narrowing, onecan increase the peaking amplitude, increase the peaking frequency, andreduce the roll-off factor. One can potentially adjust the parametersabove simultaneously.

While various embodiments have been described and illustrated herein, avariety of other means and/or structures for performing the functionand/or obtaining the results and/or one or more of the advantagesdescribed herein, and each of such variations and/or modifications arepossible. More generally, all parameters, dimensions, materials, andconfigurations described herein are meant to be examples and that theactual parameters, dimensions, materials, and/or configurations willdepend upon the specific application or applications for which thedisclosure is used. It is to be understood that the foregoingembodiments are presented by way of example only and that otherembodiments may be practiced otherwise than as specifically describedand claimed. Embodiments of the present disclosure are directed to eachindividual feature, system, article, material, kit, and/or methoddescribed herein. In addition, any combination of two or more suchfeatures, systems, articles, materials, kits, and/or methods, if suchfeatures, systems, articles, materials, kits, and/or methods are notmutually inconsistent, is included within the inventive scope of thepresent disclosure.

Also, various inventive concepts may be embodied as one or more methods,of which an example has been provided. The acts performed as part of themethod may be ordered in any suitable way. Accordingly, embodiments maybe constructed in which acts are performed in an order different thanillustrated, which may include performing some acts simultaneously, eventhough shown as sequential acts in illustrative embodiments.

All definitions, as defined and used herein, should be understood tocontrol over dictionary definitions, definitions in documentsincorporated by reference, and/or ordinary meanings of the definedterms.

As used herein, a “module” can be, for example, any assembly and/or setof operatively-coupled electrical components associated with performinga specific function, and can include, for example, a memory, aprocessor, electrical traces, optical connectors, software (stored andexecuting in hardware) and/or the like.

The indefinite articles “a” and “an,” as used herein in thespecification and in the claims, unless clearly indicated to thecontrary, should be understood to mean “at least one.”

The phrase “and/or,” as used herein in the specification and in theclaims, should be understood to mean “either or both” of the elements soconjoined, i.e., elements that are conjunctively present in some casesand disjunctively present in other cases. Multiple elements listed with“and/or” should be construed in the same fashion, i.e., “one or more” ofthe elements so conjoined. Other elements may optionally be presentother than the elements specifically identified by the “and/or” clause,whether related or unrelated to those elements specifically identified.Thus, as a non-limiting example, a reference to “A and/or B”, when usedin conjunction with open-ended language such as “comprising” can refer,in one embodiment, to A only (optionally including elements other thanB); in another embodiment, to B only (optionally including elementsother than A); in yet another embodiment, to both A and B (optionallyincluding other elements); etc.

As used herein in the specification and in the claims, “or” should beunderstood to have the same meaning as “and/or” as defined above. Forexample, when separating items in a list, “or” or “and/or” shall beinterpreted as being inclusive, i.e., the inclusion of at least one, butalso including more than one, of a number or list of elements, and,optionally, additional unlisted items. Only terms clearly indicated tothe contrary, such as “only one of” or “exactly one of,” or, when usedin the claims, “consisting of,” will refer to the inclusion of exactlyone element of a number or list of elements. In general, the term “or”as used herein shall only be interpreted as indicating exclusivealternatives (i.e. “one or the other but not both”) when preceded byterms of exclusivity, such as “either,” “one of,” “only one of,” or“exactly one of.” “Consisting essentially of,” when used in the claims,shall have its ordinary meaning as used in the field of patent law.

As used herein in the specification and in the claims, the phrase “atleast one,” in reference to a list of one or more elements, should beunderstood to mean at least one element selected from any one or more ofthe elements in the list of elements, but not necessarily including atleast one of each and every element specifically listed within the listof elements and not excluding any combinations of elements in the listof elements. This definition also allows that elements may optionally bepresent other than the elements specifically identified within the listof elements to which the phrase “at least one” refers, whether relatedor unrelated to those elements specifically identified. Thus, as anon-limiting example, “at least one of A and B” (or, equivalently, “atleast one of A or B,” or, equivalently “at least one of A and/or B”) canrefer, in one embodiment, to at least one, optionally including morethan one, A, with no B present (and optionally including elements otherthan B); in another embodiment, to at least one, optionally includingmore than one, B, with no A present (and optionally including elementsother than A); in yet another embodiment, to at least one, optionallyincluding more than one, A, and at least one, optionally including morethan one, B (and optionally including other elements); etc.

In the claims, as well as in the specification above, all transitionalphrases such as “comprising,” “including,” “carrying,” “having,”“containing,” “involving,” “holding,” “composed of,” and the like are tobe understood to be open-ended, i.e., to mean including but not limitedto. Only the transitional phrases “consisting of” and “consistingessentially of” shall be closed or semi-closed transitional phrases,respectively, as set forth in the United States Patent Office Manual ofPatent Examining Procedures, Section 2111.03.

What is claimed is:
 1. A method, comprising: receiving an analog signalfrom a communication channel, the analog signal being transmitted by atransmitter operatively coupled to (1) a finite impulse response (FIR)filter configured to receive a digital signal and (2) adigital-to-analog converter (DAC) configured to convert the digitalsignal into the analog signal; estimating a bandwidth of the analogsignal based at least in part on raw sampling data generated by ananalog-to-digital converter (ADC) operatively coupled to thetransmitter; and sending a signal representing the bandwidth of theanalog signal to the FIR filter so as to change at least one operatingparameter of the FIR filter.
 2. The method of claim 1, wherein sendingthe signal representing the bandwidth of the analog signal includessending the signal representing the bandwidth of the analog signal tothe FIR filter so as to change at least one of a peaking frequency or apeaking amplitude of the FIR filter.
 3. The method of claim 1, whereinsending the signal representing the bandwidth of the analog signalincludes sending the signal representing the bandwidth of the analogsignal to the FIR filter so as to: decrease at least one of a peakingfrequency or a peaking amplitude of the FIR filter in response to thebandwidth of the analog signal being greater than a predetermined value;or increasing at least one of the peaking frequency or the peakingamplitude of the FIR filter in response to the bandwidth of the analogsignal being smaller than a predetermined value.
 4. The method of claim1, wherein sending the signal representing the bandwidth of the analogsignal includes sending the signal representing the bandwidth of theanalog signal to a raised-cosine filter so as to: decrease a roll-offfactor of the raised-cosine filter in response to the bandwidth of theanalog signal being less than a predetermined value; or increase theroll-off factor of the raised-cosine filter in response to the bandwidthof the analog signal being greater than a predetermined value.
 5. Themethod of claim 1, wherein estimating the bandwidth of the analog signalincludes: performing Fourier transform of the raw sampling data togenerate unfiltered spectrum; performing digital filtering to theunfiltered spectrum to generate filtered data; and estimating thebandwidth of the analog signal based on the filtered data.
 6. The methodof claim 1, wherein estimating the bandwidth of the analog signalincludes: performing Fourier transform of the raw sampling data togenerate unfiltered spectrum; performing Savitzky-Golay filtering to theunfiltered spectrum to generate filtered data; and estimating thebandwidth of the analog signal based on the filtered data.
 7. The methodof claim 1, wherein estimating the bandwidth of the analog signalincludes: performing Fourier transform of the raw sampling data togenerate unfiltered spectrum; performing moving average filtering to theunfiltered spectrum to generate filtered data; and estimating thebandwidth of the analog signal based on the filtered data.
 8. The methodof claim 1, wherein sending the signal representing the bandwidth of theanalog signal to the FIR filter includes sending the signal representingthe bandwidth of the analog signal to the FIR filter via a networkmanagement device.
 9. The method of claim 1, wherein sending the signalrepresenting the bandwidth of the analog signal to the FIR filterincludes sending the signal representing the bandwidth of the analogsignal to the FIR filter via an in-band signal.
 10. An apparatus,comprising: a receiver configured to receive an analog signal via acommunication channel from a transmitter operatively coupled to (1) afinite impulse response (FIR) filter configured to receive a digitalsignal and (2) a digital-to-analog converter (DAC) configured to convertthe digital signal into the analog signal; an analog-to-digitalconverter (ADC) operatively coupled to the receiver and configured toreceive the analog signal; an estimator operatively coupled to the ADCand configured to estimate a bandwidth of the analog signal based on atleast in part on raw sampling data generated by the ADC; and atransmission device operatively coupled to the estimator and configuredto transmit a signal representing the bandwidth of the analog signal tothe FIR filter so that the FIR filter changes at least one operatingparameter in response to receiving the signal representing thebandwidth.
 11. The apparatus of claim 10, wherein the receiver includesat least one of a C-form pluggable generation 2-analog coherent optics(CFP2-ACO) receiver or a coherent in-phase/quadrature receiver.
 12. Theapparatus of claim 10, wherein the transmission device is configured totransmit the signal representing the bandwidth of the analog signal tochange at least one of a peaking frequency or a peaking amplitude of theFIR filter.
 13. The apparatus of claim 10, wherein: the transmissiondevice is configured to transmit the signal representing the bandwidthof the analog signal to change at least one of a peaking frequency or apeaking amplitude of the FIR filter; and the transmission device isconfigured to transmit the signal representing the bandwidth of theanalog signal to decrease at least one of the peaking frequency or thepeaking amplitude in the event that the bandwidth of the analog signalis greater than a predetermined value.
 14. The apparatus of claim 10,wherein the estimator is configured to: perform Fourier transform of theraw ADC data to generate unfiltered spectrum; perform digital filteringto the unfiltered spectrum to generate filtered data; and estimate thebandwidth of the analog signal based on the filtered data.
 15. Theapparatus of claim 10, wherein the estimator is configured to: performFourier transform of the raw ADC data to generate unfiltered spectrum;perform Savitzky-Golay filtering to the unfiltered spectrum to generatefiltered data; and estimate the bandwidth of the analog signal based onthe filtered data.
 16. The apparatus of claim 10, wherein the estimatoris configured to: perform Fourier transform of the raw ADC data togenerate unfiltered spectrum; perform moving average filtering to theunfiltered spectrum to generate filtered data; and estimate thebandwidth of the analog signal based on the filtered data.
 17. Theapparatus of claim 10, wherein the transmission device includes anetwork management device configured to transmit the signal representingthe bandwidth of the analog signal to the FIR filter.
 18. The apparatusof claim 10, wherein transmission device is configured to transmit thesignal representing the bandwidth of the analog signal to the FIR filtervia an in-band signal so that the FIR filter changes the at least oneoperating parameter in response to receiving the signal representing.19. A method, comprising: receiving an analog signal from acommunication channel; estimating a bandwidth of the analog signal,after transmission in the communication channel, based at least in parton raw sampling data generated by an analog-to-digital converter (ADC)operatively coupled to a transmitter, estimating the bandwidth of theanalog signal including: performing Fourier transform of the rawsampling data to generate unfiltered spectrum; performing at least oneof Savitzky-Golay filtering or moving average filtering to theunfiltered spectrum to generate filtered data; and estimate thebandwidth of the analog signal based on the filtered data; and sending asignal representing the bandwidth of the analog signal to a finiteimpulse response (FIR) filter, via least one of a network managementdevice or an in-band signal, to control an operation of the FIR filterso as to adaptively compensate for bandwidth narrowing of a digitalsignal, the FIR filter being configured to receive the digital signaland operatively coupled to a digital-to-analog converter (DAC)configured to convert the digital signal to the analog signal.
 20. Themethod of claim 19, wherein sending the signal representing thebandwidth of the analog signal includes sending the signal representingthe bandwidth of the analog signal to a raised-cosine filter so as to:decrease a roll-off factor of the raised-cosine filter in response tothe bandwidth of the analog signal being less than a predeterminedvalue; or increase the roll-off factor of the raised-cosine filter inresponse to the bandwidth of the analog signal being greater than apredetermined value.